A metal foil provided with an insulating layer or a dielectric layer which will be hereinafter simply referred to as “a metal foil provided with an insulating layer” wherein the insulating layer or the dielectric layer which will be hereinafter simply referred to “an insulating layer or the like”; has been used as an electronic circuit-forming material for forming various electronic circuits on printed wiring board or semiconductor mounting substrate, or as an electronic device-forming material for forming various electronic devices.
For example, such a metal foil provided with an insulating layer is manufactured by applying a coating liquid prepared by mixing a ceramic particle (insulating filler) and a resin composition on the surface of a metal foil, followed by curing the resin composition by means including drying and heat treatment. The insulating layer or the like composed of an inorganic material and an organic material formed by the method is popularly referred to as a composite type material.
Note that in such a metal foil provided with an insulating layer used as an electronic circuit-forming material or an electronic device-forming material being thin and having a smooth surface on the insulating layer has been required. However, when a composite-type insulating layer is provided as a thin layer, insulating filler particles may project not uniformly from the surface of the insulating layer and it makes the surface of the insulating layer rough.
For example, Patent Document 1 discloses a technology to make the surface of the insulating layer smooth in which the surface of the insulating layer is pressed with a surface smoothing unit called “a mold” after the coated film is applied on the surface of a base material to make ceramic particles projecting from the coated film get back inside of the coated film, and thus the surface of the finished insulating layer is made smooth. In the present application, the words “coated film” is a film formed on the surface of a base material (inclusive of a metal foil) by applying a coating liquid on the surface of the base material. Next, by drying the coated film followed by semi-curing or full-curing the dried coated film through heat treatment, the insulating layer is finished.